There are a lot of arithmetic conditions for which C could generate special code. There are div_t-related functions for the other direction. I for one would like a good way to obtain, using some Standard C coding pattern, fast "carry" for multiple-precision integer arithmetic.
Several places in support functions, I have coded unusually to avoid wrap-around etc. I bet you could devise something like that for (unsigned) multiplication.
Several places in support functions, I have coded unusually to avoid wrap-around etc. I bet you could devise something like that for (unsigned) multiplication.